COMHAIRLE CONTAE ÁTHA CLIATH THEAS
SOUTH DUBLIN COUNTY COUNCIL
MEETING OF LUCAN AREA COMMITTEE
Tuesday, November 23, 2010
HEADED ITEM NO. 3
Initiate a Comprehensive Analysis of Traffic Congestion of the Road Network in Lucan South of N4
This report has arisen from the Area Committee meeting in June 2010 wherein it was decided that further report would issue to the Area Committee in relation to traffic congestion in the South Lucan area.
The Council does not have the resources to provide a detailed analysis of the traffic pattern as requested in the recent motion at this time. However, in an effort to address the issue and identify the scope of such analysis a comparison has been made as an example from extracts from the following
(a)The traffic assessment of the EIS for the Outer Ring Road Scheme, (b) Traffic counts carried out as part of the Lucan Luas scheme in 2008 and(c) the current projected flows from the traffic movements carried out at a number of junctions in the area through analysis of current movements at a number of the junctions where the MOVA system operates are attached herewith. These will show the complexity of the information available and the incompatibility of conclusions that can be drawn from such information.
By way of explanation the following should be noted:
(1) The MOVA system stands for Microprocessor Optimised Vehicle Actuation. Designed by TRL (Transport Research Laboratory) in the UK during the 1980s, it is now a very well established strategy for the control of traffic light signals at isolated junctions – i.e. junctions that are uncoordinated with any neighbouring signals.
It is particularly useful at
(a) Sites with high traffic flow, particularly where there are seasonal or intermittent traffic flow fluctuations.
(b) Sites experiencing capacity difficulties under VA control with congestion on one or more approaches.
(c) Where additional capacity is required it allows pedestrian facilities and safer staging structures to be introduced.
(d) Where more than one junction is situated too close to be considered as isolated, there are ways in which two or more junctions can be linked by the use of MOVA control.
Although designed for isolated junctions, a number of linked schemes have also been installed by SDCC particularly in the South Lucan area. South Dublin County Council has over 20 sites on MOVA.
This traffic signal technology provides efficient control of the junctions ensuring the capacity of a junction or a set of junctions is maximised.
MOVA is designed to cater for the full range of traffic conditions, from very low flows through to a junction that is overloaded. For the major part of the range - before congestion occurs, MOVA operates in a delay minimising mode; if any approach becomes overloaded, the system switches to a capacity maximising procedure. MOVA is also able to operate at a wide range of junctions, from the very simple ‘shuttle-working’, to large, multi-phase multi-lane sites.
The system also provides for optimum cycle times for vulnerable road users.
(2) The ORR EIS is based on the DTO Saturn model, this is a forecasting and estimating tool that uses previous traffic flows and expected growth factors. It is useful for selecting the best road and other transport options to be provided in a scheme and provides an estimation of future traffic flows for the expected growth factors. As with any model it is the closest virtual version of reality that can be provided without actually providing the real scheme.
The figures produced in the EIS are demand volumes estimated by the model and not the capacity that the constructed junctions will have.
Since the completion of the scheme several other, but not all, transportation elements included in the model have been provided, N4 and M50 upgrades are complete, but not the Lucan Luas and Metro North or the Hazelhatch dart line.
(3) Some traffic counts have been carried out since the ORR scheme was completed. Most notably junction surveys were carried out by the RPA as part of their Luas Line F (Lucan) evaluation.
These included the junctions on the proposed route along the Outer Ring Road The number of vehicles arriving and leaving each arm of each junction, traffic flows between each junction and capacity of traffic provided by each junction have also been identified.
Limited data is also available from the signal controllers along the Outer Ring Road which has provided data on signal throughput.
Care should be taken in the use of this data as it requires considerable expertise to carry out detailed analysis and involves more than simple statistical analysis and comparison with actual and theoretical flows. The delay in the delivery of public transport infrastructure gives rise to anomalies when comparing actual with theoretical traffic flows as the modeling assumptions made are not comprehensively represented in the current existing road network. For this reason it is not correct to compare actual traffic flows with those figures projected in the EIS reports.
In an urban context the capacity of a road network is constrained and determined by the operational capacity of its junctions. In assessing the operational performance of junctions a number of criteria have been established to evaluate performance. These are identified as
- The maximum Ratio of Flow to Capacity (RFC) of any normal approach to a junction which should ideally not exceed 90% of its theoretical capacity.
- Average maximum acceptable delay per vehicle at a junction should not exceed 2 minutes.
The junctions in Lucan south area are under constant review with regard to traffic flows as these traffic lights are vehicle activated to provide maximum through of traffic in accordance with set criteria relating to road hierarchy and those mentioned above.
The vehicle activated traffic lights system provided contribute greatly to the efficient movement of traffic and maximisation of capacity.
It should be borne in mind, however, that it is not possible to cater for all traffic flows at each junction at all times. Traffic congestion is a part of life and well never be removed fully, particularly when the school run continues to be the major contributor to peak traffic flows.
Prior to ORR opening severe congestion was experienced especially in south Lucan this was instantaneously resolved with the opening of the ORR. The removal of this congestion eventually permitted previously suppressed trips to be undertaken as modeled. There was a brief window where the provision of the new road permitted a very high level of service prior to the establishment of normal traffic flows and acceptable congestion. This was interfered with during the construction of the M50 upgrade and the N4 upgrade and it will take time to settle down. Traffic counters are only now being reinstated on the major routes and these will not be fully operable in 2010.
“Acceptable congestion” is the level of congestion that drivers will tolerate and still choose to drive a busy route. This congestion can be very high where alternatives such as public transport are difficult to access. In this case bus corridors have been provided to encourage the switch to public transport as far as possible on the ORR and N4 route. In the absence of additional public transport initiatives being provided or delayed due financial constraints these junctions have a finite capacity and cannot cater for all traffic flows.
The completion of the Lucan Bypass scheme has altered the flow of traffic in the Lucan area. e.g in the evening, traffic no longer avoids the queues at the old Newcastle / N4 junction and traffic now leaving the City arrives in Lucan earlier in the evening and the peak hour has been shortened . Similar changes have occurred to traffic heading in other directions.
The Traffic section continues to monitor all of the signalised junctions on the Outer Ring Road and the Newcastle road and Fonthill road in order to recalibrate and validate the signal configurations as necessary.
Example; For instance in the case of the Outer Ring Road EIS several options were modeled, the option 1(f) is the approved scheme. The figures are shown below for each section of road along and joining the Outer Ring Road. In 2008 the RPA carried out full junction counts and these are shown next to the relevant EIS figure. (The figures are extracted from section 5.8 of the Outer Ring Road EIS page 76. and RPA traffic counts with this report).
TABLE 1
| Description | Link | Direction | EIS approved scheme | RPA 2008 count |
| 9 ORR – South of Balgaddy RB | 9 | NB | 500 | |
| 10 ORR (Balgaddy – Pennyhill) | 10 | NB | 130 | 391 |
| 10 ORR (Balgaddy – Pennyhill) | 10 | SB | 490 | 704 |
| 11 ORR (Pennyhill – Ballyowen) | 11 | NB | 890 | 912 |
| 11 ORR (Pennyhill – Ballyowen) | 11 | SB | 270 | 698 |
| 12 ORR – South of N4 | 12 | NB | 870 | 931 |
| 12 ORR – South of N4 | 12 | SB | 650 | 889 |
| 24 Griffeen Avenue West of ORR | 24 | WB | 460 | |
| 24 Griffeen Avenue West of ORR | 24 | EB | 710 | |
| 25 Griffeen Avenue East of ORR | 25 | EB | 910 | |
| 25 Griffeen Avenue East of ORR | 25 | WB | 300 | |
| 26 Castle Road West of ORR | 26 | WB | 140 | 147 |
| 26 Castle Road West of ORR | 26 | EB | 500 | 321 |
| 27 Castle Road East of ORR | 27 | EB | 60 | 338 |
| 27 Castle Road East of ORR | 27 | WB | 690 | 308 |
| 28 Esker Lane | 28 | WB | 220 | 529 |
| 28 Esker Lane | 28 | EB | 690 | 335 |
| 29 Ballyowen Road | 29 | EB | 1100 | 236 |
| 29 Ballyowen Road | 29 | WB | 220 | 554 |
It should be noted that the figures are distorted as the M50 works and the N4 works have been undertaken during the 2005 to 2009 period and care should be taken when analysis of these most recent flows have been taken.
For better understanding of junction performance the EIS figures are collated on a junction by junction basis. This allows comparison between the EIS model, RPA counts and data recovered from the traffic signal controllers
TABLE 2
| AM Peak hour junction comparison | ||||
| Outer Ring Road junctions | EIS - 2019 | RPA -2008 | MOVA - 2010 | |
| Ballyowen/ Willsbrook Junction | 2450 | 2690 | 2444 | |
| Castle Road junction | 1590 | 1718 | 2099 | |
| Griffeen Road junction | 2000 | - | 2352 |
In table 2 it can bee seen that the Willsbrook junction has a similar capacity to that suggested in the Outer Ring Road EIS, but that during 2008 it was substantially higher, perhaps due to nearby major road works. Conversely the Castle road junction has substantially less possibly due to avoidance of this route and congestion noticed at the Ballyowen Willsbrook Junction. This has now reversed benefiting local movements.
In contrast Table 1 indicates a divergence of estimated and recorded traffic flows (all be it during the N4 upgrade) in any case the signal system redistributes the capacity to accommodate the realised traffic demand. Photos of the junctions operating during the morning are shown below. Please note the time stamp on the upper right corner. From the photos it can be observed that the signals clear queues on each cycle of the signals. Additional photos are available.
Extract of traffic signal controller log files
Traffic signal controller or MOVA data files. In each case the vehicle detectors data file was interrogated for the number of detections. It should be noted that the detectors are designed for signal operation and not necessarily counting of vehicles although in effect this is what they do. TOTX is the total vehicles entering the junction form all approaches. The data is selected for 9:00am as that is the hour modeled for the EIS. Full spreadsheet and data files are available with this report.
Ballyowen/ Willsbrook Junction
Table 3
| day | Hour | time | TOTX |
| 28-Sep | 09:00 | 28/09/2010 09:00 | 2510 |
| 29-Sep | 09:00 | 29/09/2010 09:00 | 2456 |
| 30-Sep | 09:00 | 30/09/2010 09:00 | 2424 |
| 01-Oct | 09:00 | 01/10/2010 09:00 | 2401 |
| 02-Oct | 09:00 | 02/10/2010 09:00 | 857 |
| 03-Oct | 09:00 | 03/10/2010 09:00 | 464 |
| 04-Oct | 09:00 | 04/10/2010 09:00 | 2443 |
| 05-Oct | 09:00 | 05/10/2010 09:00 | 2393 |
| 06-Oct | 09:00 | 06/10/2010 09:00 | 2397 |
| 07-Oct | 09:00 | 07/10/2010 09:00 | 2500 |
| 08-Oct | 09:00 | 08/10/2010 09:00 | 2473 |
| Week day average | 2444 |
Castle Road junction
Table 4
| day | Hour | time | TOTX |
| 27-Sep | 09:00 | 27/09/2010 09:00 | 2057 |
| 28-Sep | 09:00 | 28/09/2010 09:00 | 2144 |
| 29-Sep | 09:00 | 29/09/2010 09:00 | 2092 |
| 30-Sep | 09:00 | 30/09/2010 09:00 | 2207 |
| 01-Oct | 09:00 | 01/10/2010 09:00 | 2087 |
| 02-Oct | 09:00 | 02/10/2010 09:00 | 629 |
| 03-Oct | 09:00 | 03/10/2010 09:00 | 292 |
| 04-Oct | 09:00 | 04/10/2010 09:00 | 2075 |
| 05-Oct | 09:00 | 05/10/2010 09:00 | 2146 |
| 06-Oct | 09:00 | 06/10/2010 09:00 | 1982 |
| Week day average | 2099 |
Griffeen Road junction
Table 5
| day | Hour | time | TOTX |
| 28-Sep | 09:00 | 28/09/2010 09:00 | 2358 |
| 29-Sep | 09:00 | 29/09/2010 09:00 | 2332 |
| 30-Sep | 09:00 | 30/09/2010 09:00 | 2354 |
| 01-Oct | 09:00 | 01/10/2010 09:00 | 2336 |
| 02-Oct | 09:00 | 02/10/2010 09:00 | 609 |
| 03-Oct | 09:00 | 03/10/2010 09:00 | 314 |
| 04-Oct | 09:00 | 04/10/2010 09:00 | 2363 |
| 05-Oct | 09:00 | 05/10/2010 09:00 | 2401 |
| 06-Oct | 09:00 | 06/10/2010 09:00 | 2319 |
| Week day average | 2352 |